Organic electroluminescent (EL) devices have been known for over two decades. The stability and performance of the materials used in the devices are limitations that represent barriers to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs.
The organic layers in these devices are usually composed of a polycyclic aromatic hydrocarbon. Substituted naphthacenes are one class of fluorescent polycyclic aromatic hydrocarbons useful in the manufacture of EL devices. The naphthacene known as rubrene, or 5,6,11,12-tetraphenylnaphthacene, is commercially available and can be prepared by reacting 1,1,3-triphenylpropargyl alcohol with thionyl chloride and heating the resulting product in the presence of an organic hindered amine base. Naphthacenes prepared in this manner, must be subjected to extensive purification techniques such as column chromatography, re-crystallization and sublimation to render them free of impurities and of sufficient purity to be useful in EL devices. During the preparation and purification, the materials are exposed to the atmosphere and visible light. Rubrene and other naphthacenes are known to undergo a photo-oxidation reaction in both solution and in the solid state to give endoperoxides as described in for example, Dufraise et. al., Compt.Rend. (1926), 183, 101-105; Z. Physik. Chem (1927), 130, 472-479, Hochstrasser et. al., Trans. Faraday Soc., (1956), 52, 1363-1373, and Santamaria, Tetrahedron Letters (1981), 22(45), 4511-14.
Photo-oxidation is the process wherein a material reacts with atmospheric oxygen in the presence of light to give, in the case of naphthacenes, endoperoxides. The endoperoxides that contaminate rubrene or other naphthacene derivatives give rise to EL devices with unacceptable performance. Even very small amounts of endoperoxides, such as 1% or less, can cause significant problems in these EL devices. Because these naphthacene materials are very prone to photo-oxidation, precautions must be taken to eliminate the formation of the endoperoxides when the naphthacenes are being prepared, purified and stored. During the naphthacene preparation previously described, significant reduction in the formation of endoperoxides can easily be achieved by performing the reaction and purification in the dark or under subdued lighting, or under an inert atmosphere such as nitrogen, or preferably under both reduced lighting and inert atmosphere conditions. In addition, storage of the materials is another concern. Although the materials can be stored and sold in light tight containers free from both oxygen and light, once the container has left the manufacturer and is opened, the material will be exposed to the conditions that cause and accelerate the formation of the endoperoxides.
A solution to the storage of these materials would be to convert the materials into a form that would be less prone to photo-oxidation. Polymorphism is a phenomenon that some materials exhibit, allowing them to exist in different forms, often referred to as polymorphs, and which potentially can have different chemical and physical properties in the solid state. Polymorphism is well known in the pharmaceutical industry where the differences in the properties of polymorphs are utilized to increase the stability of drugs towards heat and humidity. Ideally, conversion of the polycyclic aromatic compound from a photo-oxidative unstable polymorph to a photo-oxidative stable polymorph would solve the storage problem of such materials.
Redkar et. al., U.S. Pat. No. 6,756,381; Quallich et. al., in U.S. Pat. No. 6,387,925; Imai et. al., in U.S. Pat. No. 6,140,321; and Horvath in U.S. Pat. No. 6,124,340 describe specific examples and applications of polymorphism in the pharmaceutical industry.
Redkar et. al., in U.S. Pat. No. 6,756,381 describes a preparation for polymorphic forms of 9-nitrocamptothecin (9NC). Distinct crystal forms of 9NC are obtained by crystallizing it from different solvent systems and under various conditions, and are further characterized using various analytical methods. For example, a particular crystal form of 9NC is provided which is characterizable as having, by differential scanning calorimetry, an endotherm at between 175.5° C. and 177.2° C., and an exotherm at between 181.7° C. and 183.7° C. Various polymorphs of 9NC were prepared and their solubility was studied in different solvents.
Quallich et. al., in U.S. Pat. No. 6,387,925 describes the two crystalline polymorphic Forms A and B of (2-benzhydryl-1-azo-bicyclo[2.2.2]oct-3-yl)-(5-isopropyl-2-methoxybenzyl)amine citrate monohydrate. The pharmaceutical composition containing at least 1 of these polymorphs has advantageous stability for formulation to treat acute emesis in patients receiving chemotherapy. Citric acid was added to the free base of said compound in iso-propanol and the mixture was stirred until crystallization started. Filtration and granulation gave the anhydrous citrate salt (Form A). This Form A was dissolved in methanol and the solution concentrated to give Form B. Form A of the citrate salt is hygroscopically stable which is advantageous in overcoming pharmaceutical formulations problems due to weight changes. Form B exhibits similar properties but not as pronounced.
Imai et. al., in U.S. Pat. No. 6,140,321 describes four polymorphs of the drug donepezil-HCl for pharmaceutical uses, which are stable against heat and humidity when compared to the previously known form. They are prepared by dissolving the free base in ethanol followed by treatment with diisopropyl ether. The four polymorphs are formed by varying the time before filtration, and are characterized by peaks in their x-ray powder diffraction patterns, and absorption peaks in their infrared absorption spectra.
Horvath in U.S. Pat. No. 6,124,340 describes an invention related to a novel form of the HMG-CoA reductase inhibitor fluvastatin, more specifically to a highly crystalline form of sodium fluvastatin, referred to as sodium fluvastatin Form B. The previous form of sodium fluvastatin was very susceptible to degradation below pH 8 and also showed sensitivity to heat and light. Horvath describes several methods for preparing sodium fluvastatin Form B, which has been found to exhibit reduced degradation below pH 8 and also, improved sensitivity towards heat and light.
There is no indication in the art that polycyclic aromatic compounds can be converted to polymorphic forms that are more stable to photo-oxidation for use in the long-term storage of these materials.
The device stability and luminance performance of polycyclic aromatic compounds in EL devices in general, tends to improve when fabricated from materials with high purity and free of endoperoxides. There is a continuing need in the EL industry for new procedures for the preparation of high purity polycyclic aromatic hydrocarbons but in particular naphthacene compounds which can be stored for prolonged periods of time with little or no photo-oxidation. Devices fabricated from naphthacenes with small amounts of endoperoxides give poor performing EL devices and limit the applications of these EL devices.
The problem to be solved therefore is to provide a simple process that would significantly reduce or eliminate the propensity of polycyclic aromatic compounds to undergo photo-oxidation. Such a process should yield polycyclic aromatic hydrocarbons more robust in their exposure to light and oxygen under storage conditions, significantly reducing the formation of endoperoxides, and thus leading to EL devices that give satisfactory performance.